The present invention relates to wound dressings of the type which prevent a wound from drying out in order to promote moist wound healing.
The fluid produced by the human body and which accumulates in a wound is commonly referred to as "wound exudate". Wound exudate comprises a mixture of substances including water, salts, proteins, and bacteria. Various studies have demonstrated that, if kept moist, wounds tend to heal faster, especially in the final stages of healing. That form of healing is called "moist wound healing".
Traditional dressings prevented the escape of moisture (water) in liquid form, but the moisture tended to escape in vapor form at a rate which caused the wound to dry out too rapidly for moist wound healing to occur. Therefore, dressings have been designed to minimize the escape of moisture vapor as well as moisture liquid.
For example, a dressing 1 depicted in FIG. 5 comprises a backing sheet 2 coated with an adhesive layer, an absorbent pad 4, one side of the pad being adhered to the adhesive, and the other side carrying a liquid permeable net 5. The backing sheet is formed of a semi-occlusive, liquid and bacteria impermeable, polyurethane film. Once the dressing has been applied to a patient's skin S, the escape of moisture liquid is prevented, whereas moisture vapor can escape through the film 2 as demonstrated by arrows 6 and 7. The rate of that escape is controlled by the amount of resistance offered by the adhesive layer (not shown) and the semi-occlusive film 2. The rate of moisture escape, i.e., the so-called moisture vapor transmission rate (MVTR) of the dressing, is a function of a number of factors, such as the type and thickness of the adhesive layer, and the type and thickness of semi-occlusive film which is used. The MVTR characteristic of the dressing represents a rate of moisture transmission per unit area of the dressing for 24 hr., so the overall moisture loss per day of the dressing can be determined by multiplying the MVTR by the size of the area through which vapor can escape.
The above-described dressing 1 effectively controls the rate of vapor escape, since all of the escaping vapor must pass through the semi-occlusive film 2. However, the dressing suffers from certain drawbacks, most notable of which is a considerable difficulty in handling the dressing during application. That is, the film 2 is exceedingly thin and flexible and frequently becomes doubled up and stuck to itself, whereupon it becomes useless. Many of those types of dressings become ruined in the process of being applied and must be discarded.
Another conventional type of dressing 10, depicted in FIG. 6, avoids the above-described handling problem. That dressing 10 comprises a backing sheet 12 formed of a stretchable, liquid permeable fabric, an absorbent pad 14 adhered to an adhesive layer covering one side of the backing sheet 12, and a liquid permeable net 18 disposed over the pad 14. The stretchable fabric 12 is easier to handle than the film 2 of the dressing depicted in FIG. 5, thus making it less likely that the dressing will be ruined in the process of being applied. In order to resist the escape of moisture vapor, a semi-occlusive (i.e., vapor permeable, bacteria and liquid impermeable) polyurethane film 28 extends through the center of the pad 14. Some moisture vapor escapes through the film 28 (see arrow 6A), so its rate of escape is controlled by the film. However, moisture vapor is also able to bypass the film 28 (see arrow 7A) and thus escapes in an essentially uncontrolled manner. Consequently, the MVTR of the dressing is only partially determined by the adhesive-coated semi-occlusive film. The MVTR of the dressing 10 is not that of the adhesive-coated film 12, but is greater, due to the ability of moisture vapor to bypass the film and escape at a faster rate than the moisture vapor passing through the film. This means that different sized dressings designed according to FIG. 6 will not have the same MVTR because as the dressing size decreases, the size of the vapor escape area surrounding the periphery of the pad 14 represents a greater percentage of the overall vapor escape area of the dressing. Consequently, the MVTR varies inversely with the size of the dressing.
Therefore, it would be desirable to provide a dressing which exhibits a uniform MVTR from one size to the next and yet which is easier to handle than the dressing shown in FIG. 5.
Another area of concern in connection with wound dressings relates to the leakage of wound exudate. Two major causes or leakage are shearing (rubbing) forces applied to the dressing, resulting in premature removal of the dressing from the wound, and saturation of the pad with wound exudate resulting in leakage from the edges of the dressing. Leakage of wound exudate is of concern, because such leakage can expose others to infection and require that bed clothing be laundered more frequently.
Therefore, an important characteristic of the dressing is its ability to stay on the patient when rubbed against bed sheets. The main physical characteristics which will determine a dressing's ability to stay on is the strength of adhesion to the skin and the shear forces generated as the dressing is rubbed against the surface. A high (strong) adhesion will ensure that the dressing stays in place, but it will likely cause discomfort or damage to the skin when the dressing is removed. A way to enable the adhesive strength to be reduced is to decrease the friction forces which must be resisted by the adhesive. That is done by lowering the coefficient of friction of the dressing's external surface. Plastic film dressing can have lower frictional coefficients than non-woven fabrics, so the dressing according to FIG. 6 has a greater coefficient of friction than the dressing according to FIG. 5 and will generate higher shear forces when rubbed against a surface.
Therefore, it would be desirable to provide a wound dressing which not only has a uniform MVTR from one size to the next and is easy to handle, but which also tends to minimize the generation of shearing forces.